1. Field of the Invention
The present invention relates to a diode which requires a high breakdown voltage and a quick reverse recovery characteristic, for example, a freexe2x80x94wheeling diode, a voltage clamp diode or the like which is to be used together with a high breakdown voltage power semiconductor device such as an IGBT (Insulated Gate Bipolar Transistor), a GCT (Gate Commutated Turnxe2x80x94off Thyristor) or the like.
2. Description of the Background Art
FIG. 8 is a view showing a sectional structure of a conventional diode. The diode comprises an N layer 501 formed in a semiconductor substrate such as silicon, a P layer 502 formed adjacently to the N layer 501, an N+ layer 503 formed adjacently to the N layer 501 on the opposite side of the P layer 502 and having a higher impurity concentration than the impurity concentration of the N layer 501, an anode electrode 504, and a cathode electrode 505. The N+ layer 503 is provided to increase a carrier concentration, thereby making the diode thinner. The N+ layer 503 is not provided in the vicinity of the P layer 502 but on the cathode electrode 505 side in order to effectively increase a carrier concentration except a portion to be depleted.
When a reverse bias voltage is applied, by the instantaneous switching of an external circuit, to the diode in which a current flows forward, the magnitude of the current is reduced to 0, and furthermore, a great current flows in an opposite direction for a certain period. This is a transient phenomenon caused by the movement of minority carriers stored in the diode. A reverse current decreases with a decrease ratio comprising, as parameters, a value of a reverse bias voltage to be applied and a value of an inductance of the external circuit and continuously flows until a concentration of excess carriers in the vicinity of a PN junction is reduced to a certain concentration or less to form a depletion layer. A value obtained by integrating a product of the reverse current and the reverse bias voltage value with respect to time is an energy loss consumed during a reverse recovery operation.
In order to enhance the reverse recovery characteristic, a proton is often irradiated in the vicinity of a PN junction formed on a boundary surface of the N layer 501 and the P layer 502. Consequently, a recombination center of a carrier is formed and a life time of the carrier is controlled to be shortened. Moreover, the diffusion of a heavy metal, the irradiation of electron beams and the like are carried out over the whole semiconductor substrate and the life time of the carrier is similarly controlled. If the life time of the carrier is shortened, the number of the minority carriers to be stored is decreased. Consequently, the value of the reverse current can be controlled.
In the conventional diode described above, the current can be reduced during the reverse recovery operation by controlling the life time of the carrier in the vicinity of the PN junction to be shortened and the energy loss can be controlled.
In the case in which a great reverse bias voltage value is applied during the reverse recovery operation, however, there is a problem in that the voltage to be applied to the diode is rapidly oscillated to easily make EMI (Electroxe2x80x94Magnetic Interference) noises to cause a malfunction of peripheral electric machinery and apparatus. It is supposed that the variation in the voltage is caused in the following manner.
More specifically, during the reverse recovery operation, the diode has a capacitance component comprising, as parameters, a distance of a depletion layer and the number of excess carriers and a resistance component comprising, as parameters, a value of an applied reverse bias voltage and values of a leakage current and a current flowing by the movement of the excess carriers. By adding an inductance component of an external circuit for applying a reverse bias voltage to the capacitance component and the resistance component of the diode, an LCR series circuit is formed. The capacitance component and the resistance component of the diode are changed on a time basis with an enlargement of the depletion layer. Particularly the resistance component is rapidly increased when the depletion layer is generated so that a concentration distribution of the excess carriers is changed. If the natural oscillating conditions of the LCR series circuit are met by the change in the capacitance component and the resistance component, a voltage oscillation is caused.
Moreover, when the depletion layer reaches the N+ layer 503, the resistance component is changed suddenly. Consequently, it is also supposed that a trigger is generated to cause a voltage oscillation.
In order to solve the above-mentioned problems, it is an object of the present invention to implement a diode in which an energy loss is controlled during a reverse recovery operation and an applied voltage is oscillated with difficulty even if a reverse bias voltage has a great value.
A first aspect of the present invention is directed to a diode comprising a semiconductor substrate having a first main surface and a second main surface opposed to the first main surface, a first semiconductor region of a first conductivity type which faces the first main surface and is formed in the semiconductor substrate, a second semiconductor region of a second conductivity type which faces the second main surface and is formed in the semiconductor substrate adjacently to the first semiconductor region, at least one third semiconductor region of the first conductivity type formed in a position on the second semiconductor region facing the second main surface that a depletion layer extended from a boundary surface of the first and second semiconductor regions with application of a reverse bias voltage does not reach, a first electrode formed on the first main surface, and a second electrode formed on the second main surface.
A second aspect of the present invention is directed to the diode according to the first aspect of the present invention, wherein the second semiconductor region has a higher impurity concentration in second main surface side than in first semiconductor region side.
A third aspect of the present invention is directed to the diode according to the first or second aspect of the present invention, wherein a diameter of a portion of the third semiconductor region which faces the second main surface is approximately 400 xcexcm or less.
A fourth aspect of the present invention is directed to the diode according to any of the first to third aspects of the present invention, wherein a total area of a portion of the third semiconductor region which faces the second main surface is approximately ⅖ or less of an area of the second main surface.
A fifth aspect of the present invention is directed to a diode comprising a semiconductor substrate having a first main surface and a second main surface opposed to the first main surface, a first semiconductor region of a first conductivity type which faces the first main surface and is formed in the semiconductor substrate, a second semiconductor region of a second conductivity type which faces the second main surface and is formed in the semiconductor substrate adjacently to the first semiconductor region, at least one third semiconductor region of the first conductivity type which faces the second main surface and is formed in the second semiconductor region, a first electrode formed on the first main surface, and a second electrode formed on the second main surface, wherein a diameter of a portion of the third semiconductor region which faces the second main surface is approximately 400 xcexcm or less.
A sixth aspect of the present invention is directed to a diode comprising a semiconductor substrate having a first main surface and a second main surface opposed to the first main surface, a first semiconductor region of a first conductivity type which faces the first main surface and is formed in the semiconductor substrate, a second semiconductor region of a second conductivity type which faces the second main surface and is formed in the semiconductor substrate adjacently to the first semiconductor region, at least one third semiconductor region of the first conductivity type which faces the second main surface and is formed in the second semiconductor region, a first electrode formed on the first main surface, and a second electrode formed on the second main surface, wherein a total area of a portion of the third semiconductor region which faces the second main surface is approximately ⅖ or less of an area of the second main surface.
A seventh aspect of the present invention is directed to the diode according to the fifth or sixth aspect of the present invention, wherein the second semiconductor region has a higher impurity concentration in second main surface side than in first semiconductor region side.
According to the first aspect of the present invention, at least one third semiconductor region is formed in the position on the second semiconductor region facing the second main surface that the depletion layer extended from the boundary surface of the first and second semiconductor regions with the application of the reverse bias voltage does not reach. Therefore, a current density of a reverse current obtained during a reverse recovery operation can be increased, a resistance component of the diode can be prevented from being suddenly changed and a voltage oscillation can be prevented from being generated.
According to the second aspect of the present invention, the second semiconductor region has a higher impurity concentration in second main surface side than in first semiconductor region side. Therefore, a carrier concentration in the second semiconductor region can be increased except a portion to be depleted. Thus, the diode can be made thinner. Furthermore, the carrier concentration is increased. Consequently, the reverse current obtained during the reverse recovery operation can be more increased between the second semiconductor region and the third semiconductor region. Thus, the sudden change of the resistance component of the diode and the generation of the voltage oscillation can be prevented more reliably.
According to the third aspect of the present invention, the portion of the third semiconductor region which faces the second main surface has a diameter of approximately 400 xcexcm or less. Therefore, it is possible to prevent the voltage oscillation without increasing a forward voltage and an energy loss produced during the reverse recovery operation.
According to the fourth aspect of the present invention, the portion of the third semiconductor region which faces the second main surface has a total area of approximately ⅖ or less of the area of the second main surface. Therefore, it is possible to effectively control the energy loss produced during the reverse recovery operation without increasing the forward voltage.
According to the fifth aspect of the present invention, at least one third semiconductor region is formed in the second semiconductor region facing the second main surface. Therefore, the current density of the reverse current obtained during the reverse recovery operation can be increased, the sudden change of the resistance component of the diode can be prevented and the generation of the voltage oscillation can be suppressed. Moreover, the portion of the third semiconductor region which faces the second main surface has a diameter of approximately 400 xcexcm or less. Therefore, it is possible to suppress the voltage oscillation without increasing the forward voltage and the energy loss produced during the reverse recovery operation.
According to the sixth aspect of the present invention, at least one third semiconductor region is formed in the second semiconductor region facing the second main surface. Therefore, the current density of the reverse current obtained during the reverse recovery operation can be increased, the sudden change of the resistance component of the diode can be prevented and the generation of the voltage oscillation can be suppressed. Moreover, the portion of the third semiconductor region which faces the second main surface has a total area of approximately ⅖ or less of the area of the second main surface. Therefore, it is possible to effectively control the energy loss produced during the reverse recovery operation without increasing the forward voltage.
According to the seventh aspect of the present invention, the second semiconductor region has a higher impurity concentration in second main surface side than in first semiconductor region side. Therefore, a carrier concentration in the second semiconductor region can be increased except a portion to be depleted. Thus, the diode can be made thinner. Furthermore, the carrier concentration is increased. Consequently, the reverse current obtained during the reverse recovery operation can be more increased between the second semiconductor region and the third semiconductor region. Thus, the sudden change of the resistance component of the diode and the generation of the voltage oscillation can be prevented more reliably.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.